1. Field of the Invention
The present invention relates to a current driving type emissive display apparatus such as an organic electro luminescence (hereinafter referred to as an xe2x80x9corganic ELxe2x80x9d) device having a thin-film transistor and a method for driving the apparatus, and more particularly, to a technique for realizing a reduction in deterioration over time or a reduction in both deterioration over time and electric power consumption simultaneously.
2. Description of Related Art
An operation of a conventional current driving type emissive apparatus such as an organic EL having thin-film transistors will be described with reference to FIGS. 16, 17, and 18.
FIG. 16 is an equivalent circuit diagram of one pixel of the conventional organic EL display device having thin-film transistors, FIG. 17 is an equivalent circuit diagram showing a matrix construction of the conventional organic EL display device having thin-film transistors, and FIG. 18 is a driving voltage diagram of the conventional organic EL display device having thin-film transistors.
Provided are: a data line 112; a first switching element 121 (hereinafter referred to as a xe2x80x9cswitching thin-film transistorxe2x80x9d) in which the source-terminal side is connected to the data line 112 and a gate electrode is connected to a scanning line 111; a holding electrode 113 for storage capacity, in which one terminal is connected to the drain terminal side of the switching thin-film transistor 121; a second switching element 122 (hereinafter referred to as a xe2x80x9ccurrent thin-film transistorxe2x80x9d), in which a gate terminal is connected to the drain terminal of the switching thin-film transistor and the source terminal is connected to a first feeder 114; and an organic EL device 135, one terminal of which is connected to the drain terminal of the current thin-film transistor and the other terminal of which is connected to a second feeder.
The switching thin-film transistor 121 controls the conduction between the data line 112 and the holding electrode 113 by the potential of the scanning line 111. That is, a scanning potential 211 controls the conduction between a signal potential 212 and a holding potential 213. Although an n-channel type thin-film transistor is employed. as the switching thin-film transistor 121 in this example, a p-channel type thin-film transistor may also be employed. In such an example the high-potential-side of the scanning potential 211 and the low-potential-side thereof are reverse to those of this embodiment.
In a pixel in a displaying condition, the signal potential 212 is high and the high potential is held at the holding potential 213. In a pixel in a non-displaying condition, the signal potential 212 is low and the low potential is held at the holding potential 213.
The current thin-film transistor 122 controls the conduction between the first feeder 114 and a pixel electrode 115 by the potential. of the holding electrode 113. That is, the holding potential 213 controls the conduction between a first feed potential 214 and a pixel potential 215. Although an n-channel type thin-film transistor is employed as the current thin-film transistor 122 in this example, a p-channel type thin-film transistor may also be employed. In such an example the high-potential-side of the signal potential 212 and the low potential thereof are reverse to those of this embodiment.
In a pixel in a displaying condition, the holding potential 213 is high, so the first feeder 114 and the pixel electrode 115 are electrically connected. In a pixel in a non-displaying condition, the holding potential 213 is low, so the conduction between the first feeder 114 and the pixel electrode 115 is interrupted.
In a pixel in a displaying condition, the current flows from the first feeder 114 through the current thin-film transistor 122 and the pixel electrode 115 to a second feeder 116, and the organic EL device 135 emits light. In a pixel in a non-displaying condition, no current flows, and the organic EL device will not emit light.
Since the first feed potential 214 is higher than a second feed potential 216, the current flows from the first feeder 114, through the current thin-film transistor 122, the pixel electrode 115, and the organic EL device 135 to the second feeder 116.
The actual operation of organic EL display apparatuses with thin-film transistors is not as simple as that described above and the devices operate under more complex relationships of voltages and currents. Similarly and qualitatively, however, the above description holds true.
FIG. 19 is a sectional view of the organic EL display apparatus having a conventional thin-film transistor. FIG. 20 is a plan view of the organic EL display apparatus having the conventional thin-film transistor. A section taken along the line Axe2x80x94A of FIG. 19 corresponds to a section taken along the line A-Axe2x80x2 of FIG. 20.
In the organic EL device 135, a current flows from a high-electric-potential-side electrode 165 of the organic EL device through a luminescent material 155 for the organic EL device to a low-electric-potential electrode 175 of the organic EL device. Although PPV, ITO, and Al are respectively employed as the luminescent material for the organic EL device 155, the material for the high electric potential side electrode 165 of the organic EL device, and the material for the low-electric-potential electrode 175 of the organic EL device in this example, other materials may also be used.
In the conventional example, in which an AC voltage is applied between the source terminal and the drain terminal of the switching thin-film transistor 121, to cause an alternating current to flow, a DC voltage is applied between the source terminal and the drain terminal of the current thin-film transistor 122 to disadvantageously cause a direct current to flow. This is due to an asymmetric configuration optimizing the materials of the high and low-potential-side in order to improve the luminous efficiency of the organic EL device 135; the organic EL device emits light because of the DC voltage being applied causing the direct current to flow. However, when a DC voltage is applied not only to the organic EL device but also to the thin-film transistor or a direct current flows, a rapid deterioration of the thin-film transistor over time will be caused.
On the other hand, an AC voltage may also be applied between the source terminal and the drain terminal of the current thin-film transistor 122. In this case, an alternating current will not flow through the organic EL device 135, but only a one-way current flow, because of the rectification of the organic EL device 135. In other words, the organic EL device 135 emits light in one direction, while it does not emit light in the other direction, so that the luminous efficiency deteriorates. Thus it requires an increase in power consumption to obtain the same amount of light emission as in the case in which a DC voltage is applied to cause a direct current to flow.
Thus, an object of this invention is to reduce deterioration over time of a switching element such as a thin-film transistor in a current driving type emissive apparatus such as an organic EL display device having a thin-film transistor. Another object of this invention is to reduce in deterioration over time of a switching element such as a thin-film transistor, and to simultaneously achieve both improvement in luminous efficiency and reduction in power consumption.
To solve the above problems, a first aspect of this invention consists of a plurality of scanning lines, a plurality of data lines, and thin-film transistors and a luminescent element formed in a manner such that they correspond to each intersection of the scanning lines and the data lines, wherein an AC voltage is applied between source and drain terminals of the thin-film transistors and wherein a DC voltage is applied between first and second terminals of the luminescent element.
In accordance with this invention, as discussed above not only reduction in the deterioration of thin-film transistors over time due to a direct current , but also improvement of the luminous efficiency of the luminescent elements can be realized.
A second aspect of the invention consists of plurality of scanning lines, a plurality of data lines, and thin-film transistors and a luminescent element formed in a manner such that they correspond to each intersection of the scanning lines and the data lines, wherein an alternating current flows between source and drain terminals of the thin-film transistors and wherein a direct current flows between first and second terminals of the luminescent element.
In accordance with this invention, as discussed above not only reduction in deterioration of thin-film transistors over time due to a direct current , but also improvement of the luminous efficiency of the luminescent element can be realized.
A third aspect of the invention is a current driving type emissive apparatus which consists of a plurality of scanning lines, a plurality of data lines, a first feeder, and a second feeder, the current driving type emissive apparatus having in correspondence with each intersection of the scanning lines and the data lines, a first switching element, a second switching element, a storage capacitor, a pixel electrode and a luminescent element, the first switching element controlling conduction between the data lines and the storage capacitor by means of a potential of the scanning lines, the second switching element controlling conduction between the first feeder and the pixel electrode by means of a potential of the storage capacitor, to thereby control a current flowing through the luminescent element arranged between the pixel electrode and the second feeder, wherein the luminescent element consists of a first luminescent element that emits light by a current flowing from the pixel electrode to the second feeder and a second luminescent element that emits light by a current flowing from the second feeder to the pixel electrode, and wherein the first and the second luminescent elements are arranged in parallel.
In accordance with the invention, as discussed above when the potential for the first feeder across the second feeder is inverted at predetermined intervals, an AC voltage is applied between the source and the drain terminals of the second switching element, causing an alternating current to flow. Then since a deterioration of the second switching element over time due to a DC voltage or a direct current can be reduced, and either of the first luminescent element or the second luminescent element can emit light, the power consumption can decrease without deterioration of the luminous efficiency.
A fourth aspect of the invention, is a current driving type emissive display apparatus which consists of plurality of scanning lines, a plurality of data lines, a first feeder, and a second feeder, the current driving type emissive display apparatus further having, in correspondence with each intersection of the scanning lines and the data lines, a first switching element, a second switching element, a storage capacitor, a pixel electrode and a luminescent element, wherein the first switching element controls conduction between the data lines and the storage capacitor by means of a potential of the scanning lines, the second switching element controls conduction between the first feeder and the pixel electrode by means of a potential of the storage capacitor, to thereby control a current flowing through the luminescent element arranged between the pixel electrode and the second feeder, wherein the luminescent element and a rectifier are arranged in parallel between the pixel electrode and the second feeder, wherein the luminescent element emits light by a current flowing from the pixel electrode to the second feeder, and wherein the rectifier is formed in a manner such that a current flows from the second feeder to the pixel electrode.
In accordance with the invention, as discussed above an AC voltage is applied between the source and the drain terminals of the first switching element and the second switching element causing an alternating current to flow, and a direct current flows between the first and the second terminals of the luminescent element. Since a luminescent element emits light by a current flowing from the pixel electrode to the second feeder, and the rectifier is constructed in a manner such that a current flows from the second feeder to the pixel electrode, deterioration of the second switching element over time due to a direct current or a DC voltage can be prevented.
A fifth a aspect of the invention is a current driving type emissive display apparatus which consists of a plurality of scanning lines, a plurality of data lines, a first feeder, and a second feeder, the current driving type emissive display apparatus further having, in correspondence with each intersection of the scanning lines and the data lines, a first switching element, a second switching element, a storage capacitor, a pixel electrode and a luminescent element, wherein the first switching element controls the conduction between the data lines and the storage capacitor by means of a potential of the scanning lines, and the second switching element controls the conduction between the first feeder and the pixel electrode by means of a potential of the storage capacitor, to thereby control a current flowing through the luminescent element arranged between the pixel electrode and the second feeder, wherein the luminescent element and a rectifier are arranged in parallel between the pixel electrode and the second feeder, wherein the luminescent element emits light by a current flowing from the second feeder to the pixel electrode, and wherein the rectifier is formed in a manner such that a current flows from the pixel electrode to the second feeder.
In accordance with the invention, as discussed above when a luminescent element emits light by a current flowing from the second feeder to the pixel electrode, and the rectifier is constructed in a manner such that a current flows from the pixel electrode to the second feeder, deterioration over time due to a DC voltage or a direct current can be reduced.
A sixth aspect of the invention is a current driving type emissive display apparatus which consists of a plurality of scanning lines, a plurality of data lines, a first feeder, and a second feeder, the current driving type emissive display apparatus further having, in correspondence with each intersection of the scanning lines and the data lines, a first switching element, a second switching element, a storage capacitor, a pixel electrode and a luminescent element, wherein the first switching element controls conduction between the data lines and the storage capacitor by means of a potential of the scanning lines, wherein the second switching element controls conduction between the first feeder and the pixel electrode by means of a potential of the storage capacitor, to thereby emit light by a current following through the luminescent element, which is arranged between the first electrode connected to the pixel electrode and the second electrode connected to the second feeder, the current driving type emissive display apparatus further consisting of of a first rectifier constructed so as to cause a current to flow from the pixel electrode to the first electrode, a second rectifier constructed so as to cause a current to flow from the second electrode to the pixel electrode, a third rectifier constructed so as to cause a current to flow from the second feeder to the first electrode, and a fourth rectifier constructed so as to cause a current to flow from the second electrode to the second feeder, and wherein the luminescent element emits light by a current flowing from the first electrode to the second electrode.
This invention enables reduction in deterioration of the second switching element over time due to a DC voltage and a direct current.
A seventh aspect of the invention, relates to a method for driving a current driving type emissive display apparatus as discussed above wherein a voltage between the first feeder and the second feeder is inverted at predetermined intervals.
This invention can realize a direct current caused to flow through a luminescent element, and simultaneously an AC voltage applied between the source and drain terminals of the second switching element causing an alternating current to flow. Additionally, there can be realized a decrease in the deterioration of the second switching element over time due to a DC voltage or a direct current, and simultaneously, the luminous efficiency can be improved and by emitting a luminescent element during both inverting periods of the potential between the first feeder and the second feeder, the power consumption can be reduced.
An eighth aspect of the invention, relates to a method for producing a current driving type emissive display apparatus discussed above, wherein at least one of the rectifiers, the first rectifier, the second rectifier, the third rectifier, and the fourth rectifier consists of a switching element for rectification connected to a gate, a source, or a drain terminal, and wherein the switching element for rectification is formed simultaneously with at least one of the first and the second switching elements.
A ninth aspect of the invention, is a current driving type emissive display apparatus discussed above wherein at least one of the rectifiers, the first rectifier, the second rectifier, the third rectifier, and the fourth rectifier is formed by a PN junction or a PIN junction, and formed simultaneously with at least one of the first and the second switching elements.
This configuration can realize a reduction in the deterioration of the second switching element over time due to a DC voltage or a direct current, and also improvement of the luminous efficiency and reduction in the power consumption by permitting a luminescent element to emit light during both periods of inversion of the potential between the first feeder and the second feeder, without an additional producing process.